The present invention relates to microbiological test arrays suitable for use in automated analyzers employing a carrier to transport such arrays between various functional stations. More particularly, the present invention provides means to eliminate unwanted air bubbles from interfering with optical measurements performed on liquids contained in microwells within the array.
Various types of tests related to patient diagnosis and therapy can be performed by analysis of a biological sample. Biological samples containing the patient""s microorganisms are taken from a patient""s infections, bodily fluids or abscesses and are typically placed in test panels or arrays, combined with various reagents, incubated, and analyzed to aid in treatment of the patient. Automated biochemical analyzers have been developed to meet the needs of health care facilities and other institutions to facilitate analysis of patient samples and to improve the accuracy and reliability of assay results when compared to analysis using manual operations. However, with ever changing bacterial genera and newly discovered antibiotics, the demand for biochemical testing has increased in both complexity and in volume. Because of these greater demands in conjunction with the expense and scarcity of floor space within health care institutions and the pressure to provide clinical results at lower costs, it has become important to simultaneously perform various types of biochemical tests within a highly automated and compact analyzer that operates with minimal clinician attention using cost-effective techniques.
An important family of automated microbiological analyzers function as a diagnostic tool for determining an antibiotic effective in controlling growth of the microorganism. In performing these test, in vitroantimicrobic susceptibility patterns of microorganisms isolated from biological samples are ascertained. Such analyzers have historically placed selected biochemicals into a plurality of small sample test wells in panels or arrays that contain different antimicrobics against known microorganisms in serial dilutions. Minimum Inhibitory Concentrations (MIC) of antibiotics effective against the microorganism are determined by color changes, fluorescence changes, or the degree of cloudiness (turbidity) in the sample test wells created in the arrays. By examining the signal patterns generated, MIC analyses are performed by computer controlled microbiological analyzers to provide advantages in reproducibility, reduction in processing time, avoidance of transcription errors and standardization for all tests run in the laboratory.
The use of microbiological test trays and the techniques employed in MIC tests, also known as Antibiotic Susceptibility Testing, AST, of microorganisms are also well known. AST tests are essentially broth dilution susceptibility tests using wells filled with inoculum and a growth broth, called herein a inoculum-broth solution, and increasing concentrations of a number of different antibiotics, or antimicrobial agents. The different antimicrobial agents are typically diluted in Mueller-Hinton broth with calcium and magnesium in chromogenic panels or diluted in autoclaved water with a fluorogenic compound in fluorogenic panels. The antimicrobials are diluted to concentrations that include those of clinical interest. After incubation, the turbidity or fluorescence, generally measured using a beam of radiation passing through the solution, will be less or non-existent in wells where growth has been inhibited by the antimicrobics in those wells. The analyzer compares each test well reading with a threshold value. The threshold value is a fixed number corresponding to a certain percentage of relative absorbency or fluorescence which corresponds to clinically significant growth. The MIC of each antimicrobial agent is measured either directly as visible growth, or indirectly as an increase in fluorescence.
Important challenges that must be taken into consideration when designing cost-effective, automated biochemical analyzers include the volume of reagents required per test and the cost of the disposable test panel or array. Because they are small and may be produced using mass-production, plastic injection molding techniques, it is cost-advantageous to use small sized test devices having a number of very microwells for performing AST tests. Such small sized test devices are readily amenable to automatic handling and may be used once and disposed with minimize expense. AST test devices typically consist of a plurality of adjacent microwells aligned in some sort of an array, each microwell functioning as a reaction vessel for the above mentioned biochemical reactions involving a solid phase media and a liquid phase containing a sample to be tested. An aliquot of the sample is placed in each microwell along with appropriate antibiotic reagents. AST testing usually requires that the test trays be incubated at a controlled temperature for a period of time so that an observable reaction between the sample and reagent occurs; at predetermined time intervals, each microwell of the test tray is examined for an indication of changes in color change, turbidity, or the like.
Filling the number of microwells with the required inoculum and/or reagents presents several technical challenges that are made increasingly difficult as the size of the microwells is reduced. These challenges include providing a uniformity of fill, maintaining the integrity of solution in a microwell, minimizing the effects of air bubbles that impede test observations, etc. Efforts have been made to address these challenges along with other problems and these generally employ a vacuum technique in filling microwells within a test array via an interconnected number of micro-sized channels including the introduction of especially formed features to trap air bubbles away from solution to be optically tested.
U.S. Pat. No. 5,932,177 provides a test sample card as typically used in biochemical analysis, having a number of same-sized rectangular shaped sample wells and fluid flow by means of a plurality of through-channels which route the fluid flow of samples along both the front and back surfaces of the card. Elevated bubble traps are provided, as are integral interrupt slots for sensing card position and alignment.
U.S. Pat. No. 5,922,593 discloses a microbiological test panel having a plurality of translucent cups extending from a first side of a planar surface, and a chassis having a plurality of open-ended tubes formed in the chassis. The chassis includes a plurality of raised passage walls on a second side of the planar surface that form passageways over the openings at the bottom ends of the tubes. One end of the passageway has an opening to allow an inoculum to flow through the passageway. The chassis further comprises an air communication port formed as an open-ended tube extending from the second side of the planar surface.
U.S. Pat. No. 5,746,980 discloses a test sample card with a fluid intake port and sample wells disposed between its opposite surfaces. A fluid channel network connects the fluid intake port to the sample wells and a bubble trap is connected to at least one of the sample wells by a conduit with formed in said first surface of the card. The bubble trap is formed as a depression extending part way through the card body and is covered by sealant tape.
From this discussion, it may be seen that there remains a need for an optical testing technique that simply and inexpensively solves the challenges associated with generation of air bubbles in micro-sized test arrays used in a microbiological analyzer. In particular, there is a need for a simple and inexpensive method for minimizing optical interference caused by unwanted air within the optical reading path during antibiotic susceptibility readings in a microbiological analyzer.
The present invention meets the foregoing needs by providing a method for testing a microbiological test array having a plurality of microwells prefilled with known amounts of different antibiotics in which unwanted air is removed from the region of optical testing without resorting to use of bubble traps. The microbiological test array have a generally flat lower surface with a plurality of upwardly projecting microwells connected by a number of microchannels to an open reservoir formed in a upper surface of the test array. The reservoir has an opening to permit a liquid inoculum-broth solution to flow into each of the microwells during a vacuum filling process. During AST testing, the test array is generally xe2x80x9chorizontally orientedxe2x80x9d relative to the direction of gravity forces so that test solution within the microwells is drawn downwards and air within the microwells is forced to the uppermost portion of the test array. In this horizontal position, AST readings are conducted using an interrogating beam of radiation passing horizontally through the microwells at locations devoid of air bubbles. To achieve the generally horizontal position, the test array is typically moved so that the axis of the originally upwardly projecting microwells is rotated about ninety degrees relative to its initial alignment.